1. Field of the Invention
The present invention relates to data communication systems such as those used, for example, in digital multiplexers or packet switches.
2. Description of Related Art
Typically such a data communication system comprises a data bus to which a number of line cards are connected. Each line card is arranged to receive data from an external source and to transmit the data via the bus to a selected one of the other cards connected to the bus. The receiving line card then outputs the data via its respective line or lines.
Nowadays it is common practice to use such a system for the transmission of asynchronous data. For example a system might comprise a bus and a number of X25 line cards connected to the bus. The X25 card is designed to handle a 64 kilobit data stream with the data assembled into frames which are transmitted from the card asynchronously. Since the card operates asynchronously there is no need for a given frame of data to occupy a particular time slot and so a system incorporating such asynchronous cards functions without need of any central controller providing clocking signals. However in the absence of such a central controller it is necessary to provide some means of ensuring that data is not corrupted by two cards accessing the bus simultaneously. To this end it is known to use contention systems in which the cards transmit contention signals between each other to establish which card has the right to access the bus at a particular instant. Examples of such systems are disclosed in GB-A-2114788, GB-A-2114789 and U.S. Pat. No. 4,334,288. It has also been proposed, e.g. in EP-A-0122765, to enable a card to transmit a priority contention signal so that if the card in question and another card with lower priority attempt to access the bus within the same contention cycle the privileged card is always allowed access to the bus first.